1. Technical Field
The present disclosure relates to stabilized lithium powder.
2. Related Art
Lithium ion secondary batteries have smaller weight and higher capacity than nickel-cadmium batteries or nickel-hydrogen batteries. For this reason, lithium ion secondary batteries have been widely used as a power source for portable electronic appliances. Moreover, a lithium ion secondary battery is the major candidate for a power source to be mounted on hybrid vehicles and electric vehicles. Along with the size reduction and higher functionality of the recent portable electronic appliances, the lithium ion secondary battery for the power source of such appliances has been demanded to have further higher capacity.
The capacity of the lithium ion secondary battery mainly depends on an electrode active material. As a negative electrode active material, graphite is typically used. To meet the above demand, however, it is necessary to use a negative electrode active material with higher capacity. In view of this, metal silicon (Si) with much higher theoretical capacity (4210 mAh/g) than that of graphite (372 mAh/g) has attracted attention.
On the other hand, the use of silicon oxide (SiOx) with the cycle characteristic superior to that of metal silicon has also been examined. Silicon oxide, however, has higher irreversible capacity than metal silicon. The amount of lithium that contributes to the charging and discharging is determined uniquely by the amount of lithium in the positive electrode. Therefore, the increase in irreversible capacity in the negative electrode leads to the lower capacity of the whole battery.
According to a proposed technique, for reducing the irreversible capacity, metal lithium is brought into contact with a negative electrode in advance before the charging/discharging is started, thereby doping the negative electrode with lithium (lithium pre-doping) (for example, see Japanese Patent No. 5196118, JP-A-2010-160986, and Japanese Patent No. 2699026). In particular, Japanese Patent No. 5196118 discloses a method of doping a negative electrode with lithium by forming a lithium-containing film on the negative electrode. Further, JP-A-2010-160986 discloses a method of doping a negative electrode with lithium by having lithium particles contained in a negative electrode active material layer.
As a lithium material used for the doping, a material with higher safety has been desired because the lithium has high reactivity. In view of this, stabilized lithium powder for a lithium ion secondary battery with high safety that can be easily handled has been suggested. This lithium powder includes lithium particles with a surface covered with a film that is stable in the air (see Japanese Patent No. 2699026).
A method for fabricating a negative electrode used for a lithium ion secondary battery typically includes a step of, after forming a layer including a negative electrode active material on a current collector, pressing the negative electrode active material layer so that the negative electrode active material layer is brought into close contact with the current collector. Through this pressing step, lithium metal of stabilized lithium powder particles for a lithium ion secondary battery is exposed, and accordingly the doping of the negative electrode progresses. Therefore, as desired characteristic features of the stabilized lithium powder for a lithium ion secondary battery, not only the higher stability of lithium but also the doping characteristics for achieving the excellent battery characteristics have been desired.